Polyhydroxyl compounds have become of increasing commercial importance in various fields. They are used, for example for the manufacture of non-ionic surface active compounds, as antifreezes, moisturizers and softeners, and as starting components for synthetic resins such as polyester and polyether resins on a large industrial scale.
Polyhydric alcohols are at present obtained from naturally occurring substances such as sugar or cellulose materials or synthesized by the oxidation of petroleum derivatives.
In view of the world food situation, it is undesirable to use naturally occurring substances which can serve as carbohydrate supplies for nutrition, as raw materials for industrial products. On the other hand, due to the shortage of petroleum resources, the cost of products dependent upon petroleum is constantly rising. Moreover, the supply of petroleum products is not secured over the long term. It would therefore be desirable to find manufacturing processes for polyhydroxyl compounds based on raw materials which are independent both of petroleum and of other naturally occurring nutritional substances.
Since the work of Butlerow and Loew (Ann. 120, 295 (1861) and J. pr. Chem. 33, 321 (1886)) in the previous century, it has been known that hydroxyaldehydes and hydroxyketones are formed by the condensation of formaldehyde hydrate (the term "condensation of formaldehyde" will always be used hereinafter to mean "condensation of formaldehyde hydrate with itself") under the influence of basic compounds such as calcium hydroxide or lead hydroxide. Since formaldehyde can be obtained from coal or natural gas by way of methanol, this would in theory be a way of obtaining hydroxyl compounds without the use of petroleum. These hydroxyl compounds could then be used for the synthesis of polyhydric alcohols by electrolyte reduction or catalytic or chemical hydrogeneration.
However, in spite of many proposals to synthesize polyhydroxyl compounds by the condensation of formaldehyde, no commercially satisfactory process has yet been developed for achieving this object, because it has not hitherto been possible to synthesize mixtures of polyhydroxyl compounds having a clearly specified and reproducible hydroxyl functionality and low viscosity. Moreover, the known processes give rise to mixtures of hydroxyaldehydes and hydroxyketones which are very difficult to hydrogenate and even then only with very large quantities of catalyst.
Due to the disproportionating reaction of formaldehyde to methanol and formic acid which takes place at the same time, only moderate yields have hitherto been obtainable by the known processes, so that isolation of the aqueous or aqueous/alcoholic solutions obtained was very expensive.
It is well known that the disproportionation of formaldehyde into methanol and formic acid is powerfully catalyzed by basic compounds. As stated by Pfeil, in Chemische Berichte 84, 229 (1951), the reaction velocity of this so-called Cannizzaro Reaction depends on the square of the formaldehyde concentration while the reaction velocity of formaldehyde polyaddition (C--C linkage) depends directly on the formaldehyde concentration (Pfeil and Schroth, Chemische Berichte 85, 303 (1952)). With increasing aldehyde concentration, therefore, the ratio of the desired polyhydroxyl compounds to methanol and formic acid shifts in the direction of the unwanted compounds. In most of the processes known in the art, it is therefore proposed to carry out the condensation of formaldehyde to hydroxyaldehydes and hydroxyketones in solutions which have a low formaldehyde concentration in order to keep the quantity of by-products as low as possible. However, the water used as solvent must then be removed by distillation in order to recover the hydroxyaldehydes and hydroxyketones formed in the process. This involves high energy costs due to the heat required for evaporation of the water. Processes for the condensation of formaldehyde from dilute aqueous solutions are therefore uneconomical. Moreover, if distillation is prolonged, the hydroxyaldehydes and hydroxyketones formed undergo considerable decomposition and discoloration reactions.
It would therefore be desirable to carry out the condensation of formaldehyde from the usual commercial concentrated formalin solutions or from solutions of formaldehyde in polyhydroxyl compounds which could be used later together with the other hydroxy compounds without unwanted side reactions appearing during the condensation reaction.
A process for the preparation of aliphatic hydroxyaldehydes in which a 40% formalin solution is reacted with thallium or thallium hydroxide has been described in German Pat. No. 822,385, but this process is dangerous because of the toxicity of thallium, and the fact that thallium hydroxide is difficult to obtain. Moreover, the yields of this process are relatively low, being only 70 to 80%.
It has also been proposed to prevent the Cannizzaro reaction by reacting formaldehyde solutions with calcium or lead hydroxide in the presence of methanol, ethanol or other polar organic solvents as described in German Pat. No. 830,951 and Gorr and Wagner, Biochemische Zeitschrift, 262, 361 (1933).
This addition or organic solvents, however, reduces the formaldehyde content of the solution. The additional energy costs required for evaporating the additional solvent to isolate the hydroxyaldehydes and ketones would therefore indicate that this process is also uneconomic. Moreover, formaldehyde and lower alcohols react to form unstable semiacetals which decompose during the condensation process with spontaneous liberation of the alcohols. The consequent delays in boiling which occur if the condensation reactions are carried out at reaction temperatures above the boiling point of the alcohol give rise to violent phenomena, particularly in large reaction batches. The condensation process is somewhat dangerous when carried out on a commercial scale under these conditions.
A process for the preparation of oxy-oxo compounds in which aqueous formaldehyde solutions at concentrations of up to 30% are reacted with lead oxide or lead acetate and inorganic bases to form sugar-like compounds which reduce Fehling's solution in the cold has been described in German Pat. No. 884,794. In this process, however, the formaldehyde solution must be heated for from 7 to 8 hours. The volume/time yield is therefore not satisfactory. The relatively low yields of approximately 80%, based on the quantity of formaldehyde put into the process, are also by no means satisfactory.
A process for the preparation of hydroxyaldehydes and hydroxyketones has been disclosed in U.S. Pat. No. 2,224,910 in which the exothermic condensation of formaldehyde is regulated by the controlled addition of inorganic or organic bases to a formaldehyde solution containing lead, tin, calcium, barium, magnesium, cerium or thorium compounds as well as a compound which is capable of enediol formation, such as glucose, ascorbic acid, fructose, benzoin, glycol aldehyde, erythrose, reductose, invert sugar or condensation products of formaldehyde. Although this process makes it possible for a mixture of hydroxyaldehydes and hydroxyketones to be obtained from relatively highly concentrated formaldehyde solutions without the addition of organic solvents, it involves various disadvantages. If the reaction is carried out at a low pH, the reaction product consists mainly of hydroxyaldehyde and hydroxyketone mixtures which have a low hydroxyl functionality. Moreover, only moderate reaction velocities are obtained at low pH values, so that the volume/time yields of this variation of the process are not satisfactory. To overcome these disadvantages, it is recommended in the cited Patent Specification to start the formaldehyde condensation reaction at low pH values and then complete it at higher pH values. However, at pH values at or above 7, lead catalyzed formaldehyde condensation proceeds so rapidly, spontaneously and uncontrollably that it is impossible to obtain mixtures of hydroxyaldehydes and hydroxyketones with a reproducible distribution of components by this process because the reaction times and conditions can no longer be accurately controlled. Furthermore, it is known that hydroxyaldehydes, hydroxyketones and monosaccharides decompose in an alkaline medium at elevated temperatures to dark colored compounds partly containing carboxyl groups.
These decomposition reactions occur particularly in those methods described in U.S. Pat. No. 2,224,910 which are suggested as preferred variations, and they occur to the greatest extent when most of the formaldehyde has already reacted. Hydroxyaldehyde and hydroxyketone mixtures prepared according to the process of U.S. Pat. No. 2,224,910 therefore contain decomposition products with acid groups, are brown in color and cannot be obtained reproducibly. Moreover, hydrogeneration of these mixtures requires unecomically large quantities of Raney nickel catalyst. 30 g of Raney nickel are required to hydrogenate a quantity of hydroxyaldehyde and hydroxyketone mixture equivalent to 100 g of formaldehyde.
In addition, the product mixtures obtained by the procedure described above must always be processed by distillation to purify them and recover hydroxyl compounds with a low molecular weight. It would, however, be desirable to dispense with the distillative working up of the mixture. Since this involves additional consumption of energy and cost of apparatus. It would be preferable to prepare the product mixtures in such a way that they could be used without further distillation as soon as the water of solution has been removed. However, such colorless reaction mixtures substantially free from by-products cannot be obtained by the process known in the art.
It is therefore an object of the present invention to provide a process for synthesizing mixtures of polyhydroxyl compounds which are as far as possible free from decomposition products and which can easily by hydrogenated to polyhydric alcohols with small quantities of hydrogenation catalysts or used directly for the production of polyurethane resins. The mixtures of polyhydroxyl compounds obtained should be colorless and require no further purification and have as low a viscosity as possible.
It is also an object of this invention to control the condensation of formaldehyde so that the distribution of products in the resulting mixtures of low molecular weight polyhydroxyl compounds can be varied as desired and obtained reproducibly.